Insights into the ligand effects of rhodium catalysts toward reductive carbonylation of methanol to ethanol

Methanol reductive carbonylation to ethanol catalyzed by diphosphine ligand modified Rh-based catalysts has been studied. All the catalysts show ligand effects toward the turnover frequency and selectivity. Four Rh-diphosphine complexes were isolated and single crystals were obtained. These complexes were characterized by X-ray single-crystal diffraction, NMR, FTIR, and XPS. The X-ray crystal structure of [Rh-2(mu-I)(mu-CO)(CO)(2)(dppm)(2)](+) was reported for the first time in this work. Based on the analysis of crystal structures, we unraveled the origin of the ligand effects of rhodium catalysts toward reductive carbonylation of methanol to ethanol. The diphosphine ligand with the appropriate number of methylene groups between two phosphorus atoms can improve the catalytic activity. The steric congestion around the empty coordination site in the Rh-diphosphine complexes led to a preferential reaction of rhodium with H-2, which promoted ethanol/acetaldehyde formation.